1. Field of the Invention
The present relates to a voltage regulator circuit capable of improving overshoot of the voltage regulator.
2. Description of the Related Art
As a conventional voltage regulator circuit, there has been known a voltage regulator shown in FIG. 4. An output voltage of a reference voltage circuit 1 and a voltage resulting from dividing a voltage of an output terminal 4 by resistors 5 and 6 are compared with each other by an error amplifying circuit 2 to thereby control an output transistor 3 on the basis of an output comparison result.
If the voltage resulting from dividing the voltage of the output terminal 4 by the resistors 5 and 6 is smaller than the output voltage of the reference voltage circuit 1, an output of the error amplifying circuit 2 becomes small and the output transistor 3 is strongly biased, thereby making the voltage of the output terminal 4 large. Conversely, if the voltage resulting from dividing the voltage of the output terminal 4 by the resistors 5 and 6 becomes larger than the output voltage of the reference voltage circuit 1, the output transistor 3 is weakly biased, thereby making the output terminal voltage small. In this manner, the voltage of the output terminal 4 is kept constant.
However, in the conventional voltage regulator, because the voltage resulting from dividing the voltage of the output terminal 4 by the resistors 5 and 6 is smaller than the output voltage of the reference voltage circuit 1 at the time of turning on a power supply, the output of the error amplifying circuit 2 becomes small and the output transistor 3 is strongly biased with the result that the voltage of the output terminal 4 is going to rapidly rise up to a given voltage. In this situation, since the voltage of the output terminal 4 rapidly rises, even if the voltage of the output terminal 4 exceeds the desired constant voltage, because a reaction of the error amplifying circuit is slow as compared with a rising speed of the voltage of the output terminal voltage 4, the voltage of the output terminal 4 is kept to the desired constant voltage a certain period of time after the output terminal voltage 4 exceeds the desired constant voltage.
As described above, because the voltage of the output terminal 4 changes more rapidly as a difference between the voltage of the output terminal 4 and the desired constant voltage to be outputted from the output terminal 4 increases, there arises such a problem in that a certain period of time elapses after the output terminal voltage 4 exceeds the desired constant voltage, resulting in that the constant output voltage cannot be outputted which is a main object of the voltage regulator.
The present invention has been made to solve the above problem with the conventional voltage regulator circuit, and therefore an object of the present invention is to reduce overshoot by causing an output terminal to rise up gently to a desired constant voltage using an output transistor low in driving capability from an error amplifying circuit of a voltage regulator when a voltage difference between an output of the output terminal and the desired constant voltage is large.
In order to achieve the above object, according to the present invention, there is provided a voltage regulator circuit in which an output transistor portion has two or more transistors different in driving capability, thereby being capable of reducing overshoot at the time of starting or when there is a large difference between a voltage of an output terminal and a desired constant voltage.